Gain control arrangement



Feb. 13, 1940. w, mK 2,189,925

Gm CONTROL ARRANGEMENT Filed April 29. 1936 This invention relates tocarrier wave receivers and more particularly to automatic volume cone'PatenredFeb.13,19 40 'coNritoLjannAnG rmNr r Louis William Reinkcn,Sidcup,,E ngland, assignor to International Standard ElectricCorporation,

' ApplicationApril'29, 1936; Serial N0."76,9'40 3 'In Gr eat BritainJunel, 1935 "1c1aim..(c1.179 17i) trol arrangements therefor. Inautomatic volume control systems as usue i ally employed in carrier wavereceiversthe speed at which the gain of the receiver falls for agiven-input signal is the same as the speed at which the gain increaseswhen the signal is removed. This is unimportant, and in fact desirablewhen modulated carrier waves are being received, since in order to holdthe output level constant when the signal fades either up or down theautomatic volume control should operate rapidly andat the same rateeither on an increasing or a decreasing carrier wave On commercialreceivers which are required to receive interrupted continuous wavetransmissions, however, such a high speed automatic volumecontrol'produces the undesirable effect ,of inserting blocks of noisebetween the ele-j mentaldots and dashes of the message,as be tween eachsignal element the sensitivity or gain is rapidly increased to noiselevel.

To overcome this defect it is customary in" commercial receivers to slowdown the actionof theautomatic volume control by introducing a time'lagor'delay circuit which, while preventing the receiver gain from changingrapidly and introducing noise, has necessarily the defect of causing thereceiver output to change only slow-j ly,'so that, for example, thevolume of a dash isnot constant throughout thelength or period of itstransmission. Similarly when tuning such a receiver. to a strong carrierwave, overloading with consequent excessive-output will momentarilyoccur until the automatic volume control comes into effective operation.

According to theinvention in'an automatic volume control system for acarrier wave receiver the time lag circuit which determines the responsespeed of the automatic volume control is arranged to. be asymmetrical inspeed of op.- eration with respect to increasing and decreasingreceivergain. v

According to a further feature an impedance in the time lag circuitcomprises a device having an (asymmetric resistance characteristic,

According to another feature of the invention in a radio receiverforinterrupted continuous wave reception, an automati'cvolumecontrolsystern in which the time lag circuit which dee' termines theresponse speed of the automatic volume control is arranged to beasymmetrical with respect to increasing and, decreasing receiver gain insuch manner that the automatic volume control operates "slowly toincrease gain and quickly to decrease gain. Q f j i It will beappreciated that the arrangement according to theinvention ishighlydesirable interrupted continuous wave reception as when, say, adash commences, the gain of the "receiver 1 drops very rapidly and holdsthe output volume substantially constant for the duration of the dash.When however, the transmission of the dash is completed, the receivergain during'the space or no signal period increases only relativelyslowly, thus preventing to a great extent noise from being receiveduntilthe next dash, say, commences, whenthe gain of the receiver again fallsrapidly; In this manner intelligibility .1 v is greatly increased.

The invention will be clearly understood from the following detaileddescription and the ac .companying drawing which shows one embodiment asapplied to a superheterodynereceiver, 20,1 3

in the interest of simplicity only sufficient of thecircuit beingillustrated'to assist in a: clear appreciation of the invention.

Referring to the drawing DET2 is the second detector valve in the normalchain of signal x a reception whilst V is a pentode valvefor effect ingautomatic volume control.

The intermediate frequency voltage appliedover lead IF to the grid ofthe second detector valve DET2 is also applied to the grid of the,

automatic volume control valve V which is so" biased by means of itscathode connection to the voltage divider R3, R4 that no anode currentflows in valve V when no signals are being received (i. e., it operatesas an anode bend detecductor C on the drawing is assumed to extend i tothe cathode circuits of these valveswhich have not been shown in'ordernot to complicate the diagram. I The cathode circuit of the output valveis connected to the resistance R2 inorder to maintain a sufiicientpotential drop along this resistance for proper operation of theautomatic volume control circuit even when the cathode currentsinjresistance R2 of the controlled valves have been reduced almost tozero by the automatic volume controlling potential. i

The anode potential of the valve 40 The arrangement so far described isa normal automatic volume control circuit.

The potential drop across the resistance RI due to the anode current ofthe valve V is applied over the lead AVC through a dry plate metalrectifierY or other device having an asymmetric resistancecharacteristic to bias the grids of the appropriate controlled valves.

The rectifier Y and a blocking condenser C connected between the AVGline and ground constitute a time lag circuit, the resistance of therectifier Y determining the rate-oi charging ofi the condenser 02 whichin turn determines the rate of change of grid potential of thecontrolled valves.

The rectifier Y is so connected that its resistance to the fiow ofcondenser. current is low whent the grids of the controlled valves arebecoming increasing negative (that is the receiver gainis falling andcondenser C2 charging) and high when the grids of the controlled valvesare becoming decreas'ingly negative (that is the receiver gain isincreasing and condenser C2 is discharging).', In this manner therequired asymmetric automatic volume control is achieved and the ratioof forward to -reverse resistances of the rectifier determining thedegree of asym-' metry obtained.

Anautomatic volumeoontrol system for a reciver having at least.oneamplifier valve, comprising a control valvehaving a first resistanceand a second resistance serially connected in the plate circuit thereof,a connection from the -.cathede' of, said amplifier valve through saidsecond resistance to ground, the value of said second resistance beingsuch that a potential drop is secured sufiicient for the operation ofsaid control valve, an asymmetric resistance element connected: betweenthe plate of said con- -trol' valve-and the grid of said amplifiervalve,

and a condenser connected between the amplifier "valveside of saidasymmetric resistance and a point in the plate circuit of said controlvalve such that any, potential drop across saidfirst resistance isapplied to said condenser, whereby the gain of said receiver decreasesquickly and increases relatively slowly.

LOUIS WILLIAM REINKEN!

